There is known a method disclosed in JP-A-2006-170183 as a method of adjusting ignition timing by use of a cylinder pressure sensor. According to the method disclosed in this patent document, a combustion ratio at given timing is calculated based on an output value of the cylinder pressure sensor. In addition, combustion start timing, i.e., ignition timing in a combustion chamber is adjusted so that the calculated value may coincide with a target value. Specifically, the ignition timing is adjusted so that the combustion ratio may be 50% at eight crank angle degrees after top dead center. It is known that when the ignition timing corresponds to MET, the combustion ratio is 50% in the vicinity of 10° after top dead center. The method disclosed in the above-mentioned patent document is a method for achieving ignition at MBT by feeding back the output value of the cylinder pressure sensor. As a specific feedback, an ignition-timing calculation method is disclosed of calculating a deviation between a combustion ratio calculated from the output value of the cylinder pressure sensor and its target value and inputting the calculated deviation into a proportional-integral circuit.
A combustion state in a cylinder is directly reflected in the cylinder pressure measured by a cylinder pressure sensor. Therefore, it is possible to accurately control the combustion state in the cylinder by feeding back the combustion ratio to the setting of ignition timing as the method disclosed in the above-mentioned patent document.
However, although the advantage as described above is present, when combustion is unstable as during cold start or when a combustion state is drastically changing as during acceleration/deceleration, a combustion variation resulting from such things is also reflected in the cylinder pressure. Consequently, a value of a combustion ratio calculated from the cylinder pressure becomes variable, which is fed back to the setting of ignition timing. It is probable that this may adversely promote the combustion variation. That is to say, the method disclosed in the above-mentioned patent document has room for improvement in view of robustness, i.e., the stable control of a combustion state.